TARGETED INACTIVATION OF THE GENE PSAI ENCODING A SUBUNIT OF PHOTOSYSTEM-I OF THE CYANOBACTERIUM SYNECHOCYSTIS SP PCC-6803

The psaI gene encoding the 4 kDa polypeptide subunit (PsaI) of the pho tosystem I was cloned from the Synechocystis sp. PCC 6803, a readily t ransformable cyanobacterium. The gene is located immediately downstrea m of the psaL gene in this cyanobacterial genome and encodes a polypep tide of 40 amino acid residues with a deduced molecular mass of 4,413 Da. The deduced amino acid sequence is homologous to PsaI from other c yanobacteria and higher plants. The polypeptide has an internal hydrop hobic region with a hydroyphilic C-terminal region. The gene is presen t as a single copy in the genome. While some of its transcripts are ex pressed as polycistronic mRNAs which are co-transcribed with psaL, the other may be expressed as monocistronic mRNAs. Stable mutant strains in which the gene psaI was interrupted by a gene conferring resistance to kanamycin, were generated by targeted mutagenesis. One of the muta nts was characterized in detail. SDS polyacrylamide gel electrophoresi s revealed that the mutant strain contained other polypeptides of phot osystem I in thylakoid membranes. Growth and photosynthetic characteri stics of the mutant strain under photoautotrophic conditions were simi lar to those of the wild type, suggesting a dispensable unknown role o f PsaI for photosynthesis in Synechocystis sp. PCC 6803.